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2
Olive Biology
Hava F. Rapoport, Andrea Fabbri and Luca Sebastiani
Abstract
The olive is a medium-sized evergreen tree, which integrates a unique set
of morphological and developmental characteristics suited to the relatively
dry, rustic conditions of its Mediterranean origin. Also particular to the
olive tree are its numerous small fruits, which are rich in oil that is highly
appreciated for both flavor and health benefits. The Olea europaea species
includes both wild and cultivated forms, and both a long period of
domestication and the perseverance of wild varieties provide a range of
morphological variation, as does the developmental plasticity of this
species. This chapter reviews the general growth and taxonomy of the
olive tree and describes its vegetative and reproductive morphology and
anatomy. Basic structural features of the trunk and branches, leaves, roots,
flowers, fruits, and seeds are described. Current research is indicated for
the structures discussed, and information provided regarding adaptive
significance, environmental influences, and genetically based variability
among cultivars or between wild and cultivated genotypes.
1
H.F. Rapoport (&)
Instituto de Agricultura Sostenible, Consejo Superior
de Investigaciones Científicas (CSIC), Campus
Alameda Del Obispo, Av Menéndez Pidal s/n, 14004
Córdoba, Spain
e-mail: [email protected]
A. Fabbri
Department of Food Sciences, University of Parma,
Parco, Area delle Scienze 59/A, 43124 Parma, Italy
L. Sebastiani
BioLabs, Institute of Life Sciences, Sant’Anna
University, Piazza Martiri della Libertà, 33, 56127
Pisa, Italy
e-mail: [email protected]
1.1
The Olive Tree
Description and Habit
The olive is a medium-sized evergreen tree,
which grows natively in relatively dry, rustic
conditions with a Mediterranean climate. Its
numerous small fruits are rich in oil, which is
highly appreciated for both flavor and health
benefits. Numerous wild, cultivated, and feral
genotypes exist, particularly in the Mediterranean
Basin, where it is widely spread and emblematic
of the diet and landscape. A long history of
domestication, in which cultivars have been
© Springer International Publishing AG 2016
E. Rugini et al. (eds.), The Olive Tree Genome, Compendium of Plant Genomes,
DOI 10.1007/978-3-319-48887-5_2
13
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selected at multiple times and locations, has
produced a range of tree architecture and fruit
morphology. Tree characteristics such as vigor,
stature, and crown density are genotype related,
as is fruit size and shape. In addition, the olive
tree is highly plastic in its development, so
environmental conditions and agricultural management practices, in particular pruning and
irrigation, impact greatly on both tree and fruit
forms.
A unique set of characteristics is integrated
into olive tree morphology and development.
The olive is a woody plant, which may grow as a
tree with a large central trunk that becomes
characteristically gnarled and twisted in old trees,
or as a multi-trunk, shrubby bush-like structure.
The tree trunk and limbs have a high capacity for
forming new lateral branches from numerous
meristematic zones or axillary buds formed at the
time of leaf initiation, and suckers sprout frequently around the collar at the trunk base. The
olive tree’s high branching capacity and the
longevity of the axillary buds have further
implications for its growth habit and agricultural
management. Sprouting provides healthy
regrowth following pruning or grazing and has
been useful for vegetative propagation as well as
contributing to the long endurance, sometimes
multiple centuries, of individual trees. Heavy
branching and the accompanying formation of
numerous leaves also tend to produce a dense
canopy which, for optimum fruit production,
requires thinning to allow light penetration and
reduce disease.
The olive tree is recognized as well adapted to
the dry Mediterranean environments where it is
native. Trunk and branches, leaves, fruits, and
roots all contain structural features considered
xerophytic adaptations, which aid in reducing
water loss (see future sections). As a woody
plant, the olive is characterized by a long juvenile
period, as much as 10–15 years under natural
conditions, until the tree is capable of forming
flowers, the structures responsible for sexual
reproduction. In its reproductive behavior the
olive tree tends to relatively pronounced alternate
bearing, in which years of heavy flowering and
fruiting alternate with those with highly reduced
H.F. Rapoport et al.
crop levels. While these processes can be modified by environmental conditions and tree
developmental physiology, there is a genetic
component involved in the regulation of both the
juvenile period length and the tendency to alternate bearing, with differences expressed among
cultivars.
The fruit of the olive tree is unusual in the
plant kingdom in that oil, in the form of fatty
acids, is the major storage component. It is a
drupe, in which the fleshy, oil-containing mesocarp or pulp surrounds a substantial, highly lignified endocarp or pit (see future sections). Olive
fruits are consumed in the wild by birds and
small mammals and under domestication by
humans, principally as extracted oil but also as
pickled or processed fruit.
1.2
Taxonomy
The olive, Olea europaea L., belongs to the
Oleaceae, a medium-sized family comprising
approximately 25 genera and 600 species distributed throughout temperate and tropical
regions of the world (Besnard et al. 2009). The
plants in this family are primarily evergreen trees
and bushes along with a number of vines, many
of which produce essential oils in their flowers or
fruits. Other economically important genera of
this family are Fraxinus, (ash), which is used for
lumber, Jasminum (jasmine), Ligustrum (privet),
Phillyrea, and Syringa (lilac), utilized for ornamental purposes, but only the olive is cultivated
for its edible fruit.
Olive morphology is logically representative
of the Oleaceae family, for which it is the type
species. Oleaceae diagnostic characters are principally related to leaf and flower structure
(Heywood 1985): Leaves are simple and generally occur opposite each other along the stem, or
else pinnate, with opposite leaflets along the leaf
axis. Flowers are frequently grouped in terminal
or axillary inflorescences. The flowers are radially symmetrical and usually hermaphrodite, but
sometimes unisexual. The calyx (sepal unit) and
corolla (petal unit) are hypogenous, inserted
below the ovary, with the sepals united in a
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Olive Biology
bell-like shape and the petals sometimes united.
Generally, there are two stamens which are
inserted on the petals. The ovary is superior,
inserted above the petals and sepals, consistent
with their hypogenous arrangement. Typically,
the ovary originates from two carpels and is
composed of one to two locules, each containing
two ovules. Morphological distinction between
the cultivated variety O. europaea subsp. europaea var. europaea and the wild O. europaea
subsp. europaea var. sylvestris has been mainly
based on the small fruit and pit size of the wild
variety and is considered rather unprecise and a
potential source of error (Ganino et al. 2006).
Following a thorough revision by Green
(2002), the genus Olea is considered to include
33 species and nine subspecies. Six of these
subspecies form the Olea subsection or complex:
O. europaea, O. cuspidata, O. laperrinei, O.
maroccana, O. cerasiformis, and O. guanchica.
This complex extends through Macronesia (O.
cerasiformis, and O. guanchica), Mediterranean
(O. europaea), African (O. maroccana), and
Asian (O. cuspidata) regions. Green’s taxonomic
scheme further recognizes the close relationship
between the cultivated olive and its wild relative,
often referred to oleaster or silvestris, as two
varieties within the Olea europaea subspecies:
Olea europaea subsp. europaea var. europaea
and O. europaea subsp. europaea var. sylvestris.
Historically there have been many and differing opinions as to nomenclature and hierarchies within both the olive genus and species,
particularly because of the use of morphological
characters which often vary in relation to environmental conditions or phenological stage. For
example, certain taxonomic confusion within the
species seems to have arisen because one of the
morphological characters used for taxonomic
identification is leaf size, and the juvenile foliage
of germinated seedlings or resprouted vegetation
of heavily grazed cultivars is easily misinterpreted as the small leaves characteristic of the
wild type. Molecular techniques are proving
highly useful in clarifying taxonomic relationships in O. europaea (Green 2002; Baldoni et al.
2006; Besnard et al. 2008, 2009).
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The wild O. europaea subsp. europaea var.
sylvestris is considered to be the main wild progenitor of the cultivated olive, based on similar
morphology and ecological requirements and
ploidy level (Green 2002; Besnard and Rubio de
Casas 2016). It is generally agreed that subsequent to domestication, introgressive hybridization between cultivated and wild populations in
different locations has played an important role in
the evolution of olive cultivars (see following
chapters). There is debate, however, as to whether olive domestication was initially a single
(Besnard and Rubio de Casas 2016) or multiple
events (Díez et al. 2015).
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2.1
Vegetative Morphology
and Anatomy
Trunk and Branches
Olive tree architecture, the structural pattern of
trunk and lateral shoot growth, shows high phenotypic diversity as well as ontogenetic modification as the plant matures (Rallo et al. 2008;
Hammami et al. 2012; Ben Sadok et al. 2013).
Evaluation of cultivars and of progenies in
breeding programs have demonstrated clear
genetic influence and heritability in olive architectural traits, along with a highly plastic
response to environmental and management
conditions. Young olive plants show a prevalent
tendency to form a single-axis trunk. Lateral
shoots sprout along the trunk from one or both of
the two lateral buds present at each node. Branched and unbranched nodes are distributed vertically along the trunk in a scattered or irregular
manner, although long areas of no branching are
infrequent. Within this diffuse pattern there
appear to be relatively equal tendencies for different dominant zones of more numerous and
more vigorous branching: basitony near the trunk
base, mesotony in the central region, acrotony in
the upper or distal zone, or the absence of a
dominant zone. Branch orientation is more frequently upright (orthotropic) than horizontal
(plagiotropic), and while the majority of
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branches are straight there are often substantial
numbers, which bend upward or downward.
Olive genotypes with a drooping or pendulous
branching habit are considered to be of particular
interest for hedgerow orchard systems (Rallo
et al. 2008).
Over time, olive trees exhibit a continuous
branching hierarchy, forming second order, third
order, fourth order branches, and so on
throughout the tree canopy. The branching system is monopodial, in which the main shoot and
branch axes are maintained and their apices
continue to be active, rather than being overtaken
by lesser order lateral shoots. In older trees
branching follows a proleptic pattern, in which
new-shoot sprouting from dormant lateral buds
occurs periodically, once a year. In contrast,
young trees and regrowth following heavy
pruning often present syllepsis, where multiple
orders of branches develop in the same growth
season due to reduced bud dormancy. Sylleptic
branching in the olive has a significant genetic
component and a strong role in early tree morphology (Ben Sadok et al. 2013).
Vigor is a morphological characteristic, which
refers to overall plant growth rate and size. In
cultivated olive varieties, vigor is considered to
be an important agronomic component which is
Fig. 1 Microphotograph
of xylem tissue sampled
within a yearly ring of an
olive stem. Xylem vessels
(large and red), numerous
fibers (small and red), and
both axial and ray
parenchyma cells (blue) are
visible (photograph credit
Luca Sebastiani)
H.F. Rapoport et al.
relevant to juvenile period length and variety
choice for planting density and has noted genotypic tendencies (de la Rosa et al. 2006; Rallo
et al. 2008; Farinelli and Tombesi 2015). Olive
tree vigor may be classified qualitatively as high,
medium, or low, as in the official Olive Oil
Council cultivar passport criteria (Barranco et al.
2000), or measured as canopy volume, canopy
surface parallel to the ground, or trunk diameter
at a specified distance from the soil surface
(Rallo et al. 2005). Although trunk growth is
clearly influenced by multiple factors, a molecular marker for olive trunk diameter has been
recently identified at the seedling stage (Atienza
et al. 2014).
Young olive shoots are slender, and their
anatomy is characterized by small xylem vessels,
a large central pith, and a notable sclerenchyma
ring composed of fiber bundles and sclereids
internal to the phloem. Secondary xylem in the
olive stem and trunk is diffusely porous, in which
the relatively small-diameter vessels are distributed uniformly within the annual growth ring.
The xylem tissue (Fig. 1) is also rich in fibers
and contains a relatively low proportion of
parenchymatic cells (Salleo et al. 1985; Rossi
et al. 2013). In a 1-year-old stem, vessel diameters are typically lower than 40 μm, with 90 % in
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Olive Biology
diameter classes around 20 μm (Lo Gullo and
Salleo 1990). In mature olive plants the vessel
diameters can vary among different years and
according to water availably, indicating a strong
capacity to respond to the environmental. Rossi
and coworkers (2013) found that irrigated olive
trees formed fewer small-diameter xylem vessels
(<20 μm) than rainfed trees, while vessels with
diameters higher than 20 μm were more abundant. These authors hypothesize that the narrow
xylem vessels could contribute to olive tree tolerance of low water availability.
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The olive tree is evergreen and its leaves usually
live for two to four seasons, although older
leaves may remain on the tree longer. The small
leaves are simple, with an elongated blade (3–
9 cm long) and a very short petiole (approximately 0.5 cm). Blade shape varies from the
slightly wider and more symmetrical elliptical
form to lanceolate, where the width is greatest at
the base and blade length is greater than six times
its width. A thick, highly visible central vein
divides the leaf in half along its length and protrudes along the lower surface. The leaves are
positioned on the branches with decussate phyllotaxy: There are two opposite leaves (180°
apart) at each node, alternating at right angles
with those of the nodes above and below. Shoot
growth producing new nodes and leaves may
occur at any time of the year, depending on water
supply, temperature, and solar radiation, but
usually is only active or most active in spring and
autumn (Connor and Fereres 2005).
Olive leaf anatomy and surface characteristics
(Fig. 2) are typical of drought-resistant sclerophyllous vegetation, including small size, thick
cuticle, high stomatal density, high pubescence,
and compact mesophyll cells (Bacelar et al.
2004). The upper surface of the leaf is dark green
and glossy, covered with a thick cuticle. The
lower surface appears gray or pale in color due to
the presence of numerous multicellular trichomes
in the form of umbrella-shaped peltate scales,
forming a dense protective layer. The stomata
Fig. 2 Cryo-SEM image of frozen-hydrated young leaf
(cv. Leccino), freeze fractured transversally showing cell
types. At the figure, top, the upper epidermis (Ue) and
palisade mesophyll (Pm); in the center, the spongy
mesophyll (Sm) traversed by veins (V); and below, the
lower epidermis (Le). The lower surface is covered by
numerous overlapping trichomes (Ps, peltate scales)
(photograph credit Antonio Minnocci)
2.2
Leaves
18
H.F. Rapoport et al.
Fig. 3 Cryo-SEM image
of frozen-hydrated young
leaf (cv. Leccino), lower
surface. The trichomes (Ps,
peltate scales) and the
stomata (St) they will
eventually cover are in
early stages of development
(photo credit Antonio
Minnocci)
form only on the underside of the leaf, where
they are shielded by the peltate trichomes or
scales (Fig. 3). Internally, the upper zone of the
leaf mesophyll is composed of palisade parenchyma consisting of several layers, usually two
to three, of compact, relatively elongated cells.
The spongy mesophyll, composing the lower
portion of mesophyll, is characteristically replete
with large intercellular spaces. Directly below the
lower leaf epidermis, between that tissue and the
spongy mesophyll, is frequently a second palisade or pseudo-palisade parenchyma, consisting
of a continuous single layer of elongated cells,
although shorter and less densely packed than the
principal palisade tissue (Chartzoulakis et al.
1999; Bacelar et al. 2004; Moreno-Alías et al.
2009). Within the leaf, where the palisade parenchyma and spongy mesophyll meet, is a dense
reticulate network of secondary veins, and
throughout both mesophyll layers are numerous
long, slender branched sclereids (Arzee 1953).
These latter structures contribute to the characteristic toughness of the olive leaf, also considered a typical structural feature of leaves adapted
to dry environments.
Olive leaf size and structure vary in relation to
genotype and development, and in response to
growth conditions. Leaves of juvenile plants tend
to be smaller and to have a lower length-to-width
ratio, less peltate trichomes, and no second palisade parenchyma layer (Moreno-Alías et al.
2009). Leaves from wild genotypes are also
frequently small and more elliptical in form.
Under water stress, cell density increases for all
leaf tissues, and the cuticle is substantially
thicker (Chartzoulakis et al. 1999).
2.3
Roots
Olive tree root morphology varies with growth
conditions, including soil characteristics, nutrition, and water, and with the origin of the plant
as a seedling or cutting; however little information is available with respect to cultivar differences or genetic control. Seedlings produce a
single long, primary taproot, whereas in rooted
cuttings the multiple roots tend to branch more
profusely and closer to their junction with the
stem.
Anatomically, the olive tree root presents
attributes related to withstanding water stress and
like the shoot shows the capacity to rapidly
modify its structure in relation to water availability (Rapoport 2010; Tataranni et al. 2015).
Root hairs, the elongated lateral outgrowths of
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Olive Biology
19
3
Reproductive Structures
and Development
the epidermal cells which are responsible for
absorption, are relatively short but numerous.
The endodermis is the cell layer located within
the root between the cortex and vascular cylinder
and contains a prominent Casparian strip on its
radially oriented cell walls. The substantial suberin composition of the Casparian strip is
impermeable to water and forces the water
movement through the endodermal cell protoplasts, providing the metabolically controlled
hydraulic gates for the entry of water into the
plant vascular system. The endodermis of olive
roots is well differentiated and furthermore has
demonstrated the capacity to undergo additional
suberization in response to exposure to water (Lo
Gullo et al. 1998) and salinity stress (Rossi et al.
2015).
The external cell layers of the cortex, those
that lie directly below the epidermis, also exhibit
structural modifications to create a hypodermis or
exodermis. The hypodermal cells undergo specialized secondary wall differentiation, which
includes thickening and deposition of suberin,
acting as an additional barrier to water movement
and protecting the root from water loss. In fact,
under water-limiting conditions olive roots produce added hypodermal cell layers and increased
suberization hypodermal as well as endodermal
cell walls (Tataranni et al. 2015).
In the year prior to bloom, the inflorescences
originate as buds in the leaf axils of new shoots.
The buds (Fig. 4) are small, highly reduced
shoots composed of 4–5 nodes, and each with
two oppositely positioned leaf primordia (Fabbri
and Alerci 1999; de la Rosa et al. 2000). All of
the buds are structurally identical and basically
vegetative; that is, they do not present any morphological differences which distinguish between
vegetative and reproductive buds (Pinney and
Polito 1990; Andreini et al. 2008). Subsequent
bud fate, depending on a complex combination
of endogenous and exogenous factors, will follow one of four pathways: (1) development of an
inflorescence (reproductive bud), (2) growth of a
shoot branch, (3) continue dormant, and (4) death
and abscission. The lack of microscopically visible differences between buds with reproductive
and vegetative fates has long been an obstacle for
the investigation on olive tree flowering controls.
Molecular tools offer great potential for identifying vegetative and reproductive buds and providing a means of advancing in this area
(Amasino 2010).
Fig. 4 Olive axillary bud (left) and early reproductive
and vegetative sprouting (center and right). The axillary
bud is a relatively undifferentiated structure composed of
opposite pairs of leaf primordia. In reproductive sprouting
(center), the peduncle axis elongates, the leaf primordia
form small bracts, and inflorescence development occurs
within the bracts. Vegetative sprouting (right) begins with
elongation of the branch axis and the external leaf
primordia, which will develop into the first pair of leaves
(photograph credit Franco Castillo-Llanque)
3.1
Inflorescence and Flower
20
H.F. Rapoport et al.
Reproductive differentiation proceeds the following spring under adequate environmental
conditions in buds, which have received the necessary induction and initiation stimuli. Olive
flowers are borne on 1.5–4.0 cm long inflorescences that develop from the lateral buds or
directly at the shoot apex. The paniculate
inflorescence has a central axis or rachis, on which
individual flowers or secondary flower-bearing
axes (rachillas) arise at a series of bilaterally
symmetrical branching points. One inflorescence
may contain from 10 to 35 flowers, of which only
one or a few will set fruit. The number of flowers
in an inflorescence is influenced by the cultivar
and growing conditions, including the position of
the inflorescence on the tree and the fruiting shoot.
The olive flowers are small and have actinomorphic (regular) symmetry and hypogenous
arrangement, in which the masculine and accessory flower organs are attached in successive
whorls below the ovary base. The sepals are
fused to form a small green conical calyx at the
flower base. Within the calyx are four white
symmetrically arranged petals, joined at their
base, composing the corolla. The androecium,
the male reproductive system, consists of two
stamens, each with a relatively large, plump
bright yellow anther, which are attached to the
petal bases by a short filament. In the flower
center is the gynoecium, the female reproductive
system, a single pistil composed of a long,
two-lobed stigma, a short white style and a small
round green ovary (Fig. 5).
The elongated, two-lobed stigma surface is
composed of multicellular papillae and is classified as wet type because of the secretion it
produces, a heterogeneous substance including
carbohydrates, lipids, and proteins (Serrano et al.
2008). Below the papillae lies a funnel-shaped
receptive tissue, which is continuous with and
similar in structure to the columnar transmitting
tissue in the center of the solid style (Suarez et al.
2012). The ovary (Fig. 6) has two locules or
cavities, derived from its two-carpel origin. Each
locule contains two ovules, each joined to the
upper part of the placental separation between the
locules. Olive ovules are anatropous and during
development undergo a turning movement,
which orients the micropyle, the embryo sac’s
entranceway for the pollen tube, upward toward
the style (King 1938). Fertilization of one of the
four ovules is required for fruit development.
Within the olive ovule, macrogametophyte
differentiation follows a bisporic pattern in which
two of the four megaspore nuclei produced during meiosis abort and two survive, generating an
8-celled embryo sac (Extremera et al. 1988). The
olive ovule is tenuinucellate and unitegmic, signifying that the nucellus, the ovule tissue where
the embryo sac forms, is thin at ovule maturity
and is surrounded by only a single integument.
Full development of the ovule occurs in the three
weeks prior to flowering, at which time the
embryo sac is mature and capable of fertilization.
Differentiation is sometimes incomplete and no
embryo sac forms within the ovule, only a
Fig. 5 Microphotograph of a longitudinal section of
olive gynoecium, composed of a single stigma, style, and
ovary. The upper stigma surface has received pollen
grains. The funnel-shaped receptive tissue of the upper
style is continuous with the stigma papillae. Indicated
between circles, the mesocarp or future pulp forms the
outer part of the ovary. Within the ovary endocarp, one
ovule is visible in each of the two locules (photograph
credit Hava Rapoport)
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Olive Biology
Fig. 6 Microphotograph of a central transverse section
of olive ovary. A ring of vascular bundles, indicated by
the dotted line, lies between the ovary mesocarp (Me) and
endocarp (En). Each of the two locules contains two
ovules (Ov); in this ovary, all four ovules are fully
developed, evidenced by the round clear areas (photograph credit Ester García-Cuevas)
residual atrophied nucellus at the base of a long
micropylar canal. Such incomplete ovule development was first identified in the non-fruiting
ornamental cultivar Swan Hill, but also occurs in
fruiting cultivars to varying degrees, clearly
associated with genotype (Rallo et al. 1981;
Moreno-Alías et al. 2012). Fertilization of the
undeveloped ovules is not possible, but, usually
other, fully differentiated and competent ovules
are also present in the ovary, allowing fertilization to proceed. That factor, along with the olive
tree’s tendency to produce numerous flowers,
means that in normal, good flowering years the
presence of undeveloped ovules will not likely
cause a drop in fruit production (Rapoport and
Rallo 1991).
The olive tree sexual system is andromonoecious, with two sexually different kinds of flowers present together within the same
inflorescence: (1) hermaphroditic or bisexual
flowers, with both male and female reproductive
organs, and (2) staminate, with only male
reproductive parts. The hermaphroditic flowers
are also referred to as perfect flowers, and the
staminate flowers, imperfect. Staminate flower
development involves varying degrees of pistil
21
abortion, and a residual atrophied pistil is often
visible within the center, but stamen development and pollen production are similar in both
flower types (Cuevas and Polito 2004). The
proportion of imperfect flowers varies according
to cultivar and is also influenced by environmental conditions, such as temperature, mineral
nutrition, and water availability, and substrate
competition among flowers when flower number
is high. The critical role of substrate supply in the
differentiation of the two flower types is evidenced by the much higher starch accumulation
in perfect than staminate flowers (Reale et al.
2009). Within the inflorescence, there appear to
be preferred positions for perfect flower formation, likely related to positional priorities for
substrate supply (Cuevas and Polito 2004; Seifi
et al. 2008).
The olive anthers produce large quantities of
pollen (Fig. 7) which as well as being critical for
fertilization of the flowers is also a source of
allergic reactions. Pollination is anemophilous,
by wind, and the pollen may travel over very
large distances, a process recently confirmed by
molecular paternity testing. Olive cultivars
express varying degrees of self- and interincompatibility.
3.2
Fruit
The olive fruit is a drupe, an indehiscent fleshy
fruit composed of three principal tissues, the
endocarp, mesocarp, and epi- or exocarp, each of
which follows a very distinct form of growth and
differentiation during fruit development. Together
they form the pericarp, initiated as the ovary wall.
At bloom, the ovary contains a ring of vascular
bundles which lies between the mesocarp and
endocarp (Fig. 6), at this time composed of small
parenchymatic cells (King 1938; Rallo and
Rapoport 2001; Rosati et al. 2012). Observations
of the olive ovary cells and tissues have shown that
ovary size and cell number at bloom are highly
correlated with cultivar fruit size (Rosati et al.
2009, 2012) and that ovary size differences among
cultivars are mainly due to cell number in both the
endocarp and mesocarp (Rosati et al. 2011).
22
H.F. Rapoport et al.
Fig. 7 Cryo-SEM image
of an olive anther with
numerous pollen grains
(photograph credit Antonio
Minnocci)
The endocarp is the fruit pit, a hard, woody
structure, which surrounds the seed in the fruit
center. During fruit growth the endocarp cells
undergo the differentiation process of sclerification, in which a thick secondary wall rich in lignin
is deposited, eventually filling the entire cell and
obliterating its living contents (Fig. 8). As the
endocarp grows, sclerification is initiated in successively increasing numbers of its cells (Hammami et al. 2013), while the cells which previously
started sclerification continue to undergo lignin
deposition and cell wall thickening. Physical
hardening intensifies after maximum endocarp
size is reached, the time when the majority of
endocarp cells have initiated sclerification and can
no longer expand nor undergo division. Afterwards hardening continues, as do cell sclerification
and the associated endocarp dry weight increase
(Hammami et al. 2013; Rapoport et al. 2013).
Exterior to the endocarp is the mesocarp
(Fig. 9), or fruit pulp, the major and edible part
of the olive fruit, and the site for oil metabolism
and storage. Oil deposition, for many years
mistakenly thought to occur in the large, aqueous
vacuole of the mesocarp cells, takes place in the
cytoplasm, a process consisting of the formation
and coalescence of small oil bodies to become
increasingly larger oil droplets (Rangel et al.
1997). The mesocarp is a fleshy tissue composed
of parenchyma cells, whose capacity for cell
division and expansion drives the long-term
growth of this tissue. The highest rate and
Fig. 8 Sclerification of olive endocarp cells which
produces pit hardening: a thick secondary wall rich in
lignin is formed (upper photo) which progressively
thickens, eventually obliterating the cell contents (lower
photo) (photo credit Hava Rapoport)
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Olive Biology
amount of olive fruit mesocarp cell division
occur in the period immediately following
bloom, although some cell division continues at a
reduced rate throughout fruit development
(Hammami et al. 2011). Cultivar differences in
fruit size depend principally on cell number, not
cell size, but the seasonal pattern of cell division
intensity is consistent for olive cultivars with a
wide range of fruit size (Hammami et al. 2011).
The epicarp or exocarp, the fruit skin, constitutes the external fruit tissue. The exocarp is
principally composed by the epidermis, the
external layer of cells with its thick protective
cuticle. In an attempt to clarify different views as
to whether any or how many subepidermal cell
layers also constitute part of the exocarp, recent
studies of cell dimensions (Hammami and
Rapoport 2012) indicate that approximately five
layers of subepidermal cells, although continuous
with and similar in structure to mesocarp cells,
show structural and developmental patterns more
unified with the epidermis. Scattered across the
exocarp are lenticels, derived from ovary stomata
which become atrophied early in fruit growth and
are subsequently covered with the cuticle. The
lenticels are most visible during mature green
stages of fruit growth as light-colored specs, and
their size and frequency are characters used in
cultivar identification (Barranco et al. 2000).
Fig. 9 Microphotograph of a central transverse section
of olive fruit (cv. Manzanilla de Sevilla) 8 weeks after
bloom (left) and detail of parenchymatous mesocarp at
fruit maturity (right). At 8 weeks the mesocarp (Me) is
expanding and the endocarp undergoing sclerification;
within the endocarp the locule containing the growing
seed has expanded (*). (photo credit Ester García-Cuevas)
23
The fruits of cultivated olive varieties are 1–
4 cm long, 0.5–2 cm in diameter, and can weigh
as much as 10 g fresh weight, 2.5 g dry weight.
Fruit form ranges from spherical to ellipsoidal or
elongated, with varying degrees of asymmetry
and a rounded or pointed apex. While there is a
tendency for the cultivars with smaller fruits to
be destined for oil production and those with
larger fruits for table use, many are of double
aptitude and size is not a strict determinant of
use. Fruits of wild genotypes are commonly
much smaller (length 0.5–1.5 cm, width less than
1 cm, and weight less than 2 g) and have a much
higher proportion of endocarp as compared to
cultivated varieties.
3.3
Seed and Embryo
Within the endocarp, the olive fruit contains one,
sometimes two, or very rarely three elongated
seeds, a characteristic which seems to be associated with the cultivar. The seed coat, derived
from the single ovule integument, is thin, tough,
and permeated by numerous vascular strands.
The embryo is straight, with long, flat,
spatula-shaped cotyledons, and fills most of the
seed volume. Between the embryo and seed coat
is a small amount of endosperm, rich in starch,
proteins, and oil (Alché et al. 2006).
Immediately following fertilization the multicellular endosperm is formed and commences
rapid growth, accompanied by notable expansion
of the ovule and differentiation of ovule vascular
tissues. In contrast, embryo development is
delayed, and after approximately 3–4 weeks a
single-celled proembryo, connected to a long
filamentous suspensor, appears at the tip of the
now endosperm-filled embryo sac. Embryogenesis follows the classical sequence from multicellular embryo proper to globular and then
heart-shaped embryo, and 2 months after bloom
the two cotyledons are clearly formed. At 4–
5 months, the mature olive embryo contains a
well-differentiated root apex at the tip of a short
embryo axis, and appreciable provascular strands
and storage substances in the axis and cotyledons. Between the cotyledons, however, the
24
shoot apex is flat, or sometimes slightly domed,
with a subtending zone of intense metabolic
activity but no other visible differentiation or
plumule formation (Germanà et al. 2014).
As the fruit matures, physiological dormancy
is imposed on the seed, with factors apparently
located in the endosperm as well as the embryo
itself (Germanà et al. 2014). Thus olive seed
germination procedures require removing the
mechanical barrier of the hardened pit, and cold
stratification to remove physiological dormancy.
Among tested genotypes, optimum germination
was consistent with stratification at 14 °C for
30 days, although germination rate was highly
related to genotype (Morales-Sillero et al. 2012).
At maturity, numerous olive fruits contain
empty pits with no seed present. It appears that
embryo and subsequently seed abortion are
responsible for this phenomenon, evidenced by the
consistent observation of growing fertilized ovules
in young fruits, the occurrence of aborted embryos
at two months, and by the enlargement of one of
the ovary locules in the same manner as when a
seed is present; that is, the locule expands to
accommodate the growing fertilized ovule (Rapoport 2010). Early embryo abortion could possibly
be related to post-zygotic incompatibility factors.
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